Geophysical surveys are often used for oil and gas exploration in geophysical formations, which may be located below marine environments. Various types of signal sources and geophysical sensors may be used in different types of geophysical surveys. Seismic geophysical surveys, for example, are based on the use of acoustic waves. In some seismic surveys, a survey vessel may tow an acoustic source (e.g., an air gun or a marine vibrator) and a plurality of streamers along which a number of acoustic sensors (e.g., hydrophones and/or geophones) are located. In other surveys, acoustic sensors are located on the seabed rather than being towed. This may be desirable in situations in which floating infrastructure prevents a streamer vessel from surveying an area, when wide-azimuth illumination is needed, and/or when imaging using shear-wave data is desired, for example.
Acoustic waves generated by the survey source may be transmitted to the earth's crust and then reflected back and captured at the towed and/or seabed geophysical sensors. Data collected during a marine geophysical survey may be analyzed to locate hydrocarbon-bearing geological structures, and thus determine where deposits of oil and natural gas may be located.
Four-dimensional (4D) surveying involves surveying the same geophysical formation at different points in time, e.g., to determine changes in fluid saturation, pressure, etc. over time. Seabed sensors may advantageously reduce variations between surveys, allowing high repeatability for 4D surveying (e.g., because receivers are fixed). However, it may be difficult and/or expensive to deploy and maintain large numbers of seabed sensors, so survey techniques using relatively sparse sensor arrays may be desirable.
This specification includes references to “one embodiment” or “an embodiment.” The appearances of the phrases “in one embodiment” or “in an embodiment” do not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure.
Various units, circuits, or other components may be described or claimed as “configured to” perform a task or tasks. In such contexts, “configured to” is used to connote structure by indicating that the units/circuits/components include structure (e.g., circuitry) that performs the task or tasks during operation. As such, the unit/circuit/component can be said to be configured to perform the task even when the specified unit/circuit/component is not currently operational (e.g., is not on). The units/circuits/components used with the “configured to” language include hardware—for example, circuits, memory storing program instructions executable to implement the operation, etc. Reciting that a unit/circuit/component is “configured to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. §112(f) for that unit/circuit/component.